Build a small plugin registry using the __init_subclass__ pattern (the lighter-than-metaclass way): 1. `PLUGINS` is a module-level dict mapping `name -> class`. 2. `Plugin` is a base class. `__init_subclass__` runs on any direct subclass and registers it under `cls.name`. 3. Re-registering the same name raises `ValueError("duplicate plugin: <name>")`. 4. `lookup(name)` returns the registered class or raises `KeyError(name)`.